In an effort to provide drivers with a comprehensive view of their surroundings, vehicle manufacturers have recently proposed and marketed 360 degree vision systems which display a “bird's eye” view of the vehicle and its surroundings. Such 360 degree vision systems typically utilize four wide angle cameras, one at the front of the vehicle, one at the rear and two at the sides. The outputs of these four cameras are displayed together on single display screen to provide a 360 degree image. See, for example, FIGS. 1A, 1B, and 1C, which show a vehicle 10 and the location of cameras 12a-12d, respective camera images 14a-14d, and 360 degree image 16.
A problem arises in attempting to stitch together the aforesaid camera images into a single composite image in that each camera is not absolutely fixed in position. There are tolerances during the manufacture of the cameras and assembly into the vehicle. In addition, and more importantly, the positioning of each camera will vary over the life of the vehicle as it is driven and subjected to the rigours of the real world. Vibrations from bumpy roads and door slams, the effects of car washes and repair and replacement of various parts, as well as the movement of the pivotable vehicle side rear-view mirror housings, can all have an effect of the position (including angular orientation) of the vehicular cameras.
For this reason, the commercial 360 degree vision systems may not be seamless. Instead, to avoid having to deal with misalignment of and between the four cameras, the commercial systems basically display the images from the four cameras in four predetermined regions of the display, typically leaving buffer zones 17 between the four images as seen in FIG. 1C. In other words, the images from the four cameras are not seamlessly stitched together to provide a uniform composite 360 degree image 18 as shown in FIG. 1D, which is more visually appealing.
Typically, the cameras are calibrated when the vehicle leaves the factory production line. An end of assembly line tester may be used to project predetermined targets in a controlled environment at known distances from the vehicle. Knowing the real physical position of various markers, it is possible to define a transfer function that maps camera pixel locations to real locations, and from this determine an offset to the nominal camera position. However, this end of line testing method does not solve the problem of being able to independently calibrate the cameras in the field, where there is no controlled environment in which pre-designated markers are situated at known locations. Simply put, it is not desirable to use end-of-line assembly line calibration based on predetermined targets in a controlled environment to calibrate a vehicular camera in the field.
Each vehicular camera has six degrees of freedom, three linear (up-down, right-left, forward-backward) and three rotational (roll, pitch, and yaw). In attempting to stitch together the images from the four cameras based on predetermined demarcation lines defined with respect to nominally positioned cameras, it was noticed that changes in the three rotational degrees of freedom in particular result in a noticeable visual distortion in the composite 360 degrees image. Thus, it is particularly desired to calibrate the cameras with respect to the three rotational degrees of freedom.